1. Field of the Invention
This invention relates in general to giant magnetoresistance (GMR) heads for magnetic storage systems, and more particularly to a method and apparatus for improving soft magnetic properties of a spin valve free layer while retaining giant magnetoresistance (GMR) effects.
2. Description of Related Art
Magnetic recording systems that utilize magnetic disk and tape drives constitute the main form of data storage and retrieval in present-day computer and data processing systems. In the recording process, information is written and stored as magnetization patterns on the magnetic recording medium. Scanning a write head over the medium and energizing the write head with appropriate current waveforms accomplish this recording process. In a read-back process, scanning a magnetoresistive (MR) sensor over the medium retrieves the stored information. This MR read head sensor intercepts magnetic flux from the magnetization patterns on the recording medium and converts the magnetic flux into electrical signals, which are then detected and decoded.
However, limitations of MR sensor performance were drastically expanded by the discovery of the giant magnetoresistance (GMR) effect, also known as the spin-valve effect. In contrast to a conventional MR effect, which is based on homogeneous ferromagnetic metals or alloys, the GMR effect is present only in heterogeneous magnetic systems with two or more ferromagnetic components and at least one nonmagnetic component. Hence, a GMR head has a greater sensitivity to magnetic fields from a disk.
Accordingly, a spin valve sensor is employed by a GMR read head for sensing magnetic fields on a moving magnetic medium, such as a rotating magnetic disk. A typical spin valve sensor includes a nonmagnetic electrically conductive spacer layer between a ferromagnetic pinned layer structure and a ferromagnetic free layer structure. An antiferromagnetic pinning layer interfaces and is exchange coupled to the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90° to an air bearing surface (ABS) where the ABS is an exposed surface of the sensor that faces the rotating disk. Leads are connected to the spin valve sensor for conducting a sense current.
A magnetic moment of the free layer structure is typically oriented parallel to the ABS in a quiescent condition, the quiescent condition being where the sense current is conducted through the sensor in the absence of any signal fields. The magnetic moment of the free layer structure is free to rotate from the parallel position in response to signal fields from the rotating magnetic disk. Changes in response to field signals from the rotating disk changes the resistance of the spin valve sensor due to the angle between the magnetic moments of the pinned and free layer structures. The sensitivity of the sensor is quantified by a magnetoresistive coefficient dr/R (ΔR/R) where dr is the change in resistance of the sensor between parallel and antiparallel orientations of the pinned and free layer structures and R is the resistance of the sensor when the moments are parallel. The GMR effect operates to produce a lower resistance for parallel alignment of the pinned and free layer structures, and a higher resistance for antiparallel alignment of the pinned and free layer structures.
Several classes of soft magnetic materials have evolved for use in the construction of spin valves. Permalloy, a general term that refers to alloys of Ni and Fe, is one class used in the fabrication of spin valves due to permalloy's very small anisotropy (i.e., varying of magnetic properties along different axis) and magnetostriction characteristics. Another important design feature for spin valves is to provide a magnetic material for the free layer structure that lowers coercivity, i.e., the magnetic field necessary to switch the direction of magnetization and decrease magnetic induction to zero.
Moreover, the success of hard disk drives (HDDs) originates from these successful design features and an ever-increasing demand for storage capacity coupled with a consistent reduction in price per megabyte. Areal density (expressed as billions of bits per square inch of disk surface area, Gbits/in2) is the product of linear density (bits of information per inch of track) multiplied by track density (tracks per inch), and varies with disk radius. Improved areal density levels have been the dominant reason for the reduction in price per megabyte. High areal densities have been achieved by introducing new technology and by proportionally reducing certain key dimensions, such as the GMR head, within the HDD (“scaling”). Thus, there is a present need to reduce the free layer thickness in GMR spin valve sensor.
Current spin valve designs have free layers composed of a bilayer of CoFe and NiFe. A minimum thickness of CoFe in contact with a Cu spacer layer in the spin valve is necessary to achieve the highest GMR signal. In other words, sensitivity is increased with a reduction in the thickness of the free layer. However, to maintain acceptable sensor performance, and GMR, the bilayer material CoFe should not be reduced far below 15 Å. Hence, in reducing the bilayer structure of CoFe and NiFe to a thickness below 15 Å, the NiFe must be reduced to near zero.
The soft magnetic properties of CoFe are less attractive than NiFe and as the total thickness of the free layer is reduced, the ratio of CoFe to NiFe increases. As a result of the increased ratio, coercivity increases causing a strong resistance to change in magnetization of the bilayer structure. Thus, it is important to find a replacement for CoFe with improved soft magnetic properties, yet while retaining high GMR.
It can be seen that there is a need for providing a high quality soft magnetic material for the spin valve free layers of magnetic recording heads.
More particularly, it can be seen that there is a need for providing improved soft magnetic properties for free layers of spin valves while retaining giant magnetoresistance (GMR) effects.